Also, who has played Necromancer against Forcemaster? One would think Necromancer would have an edge with his creature having psychic immunity, but then again the Necromancer does not do well when the enemy mage is right up in his face.
I've played necromancer v. forcemaster (I was the forcemaster). It was extremely close, but the necromancer won in the end, mostly through poison damage over time and spamming drain life while running away from me. I made some poor strategic choices early on that probably contributed to this. It came down to a final showdown where I had 3 attacks (of 5,4, and 3 dice) and the necromancer was within 10 damage points of death, but I was within 2 points of death. I had initiative, and nullified the expected drain life. Unfortunately, I rolled poorly. Psychic immunity was not really a factor since my forcemaster does not use much in the way of psychic spells (more of an emphasis on force spells and enchantments). Mordok's Obelisk and Suppression orb shut down zombie swarm pretty fast.
The Altar is definitely going to be an auto-include when I get a chance to tear down the preconstructed spellbook and make my own. Don't forget that the cleric can reconstruct your walls, too, if you're planning on using those. I think they're probably not worth it if you're focusing on zombies, but with a skeleton build (which I'm working on partly just to be a contrarian), I think that they can be useful by keeping the walls/altar/Mort alive dead, but relatively intact.
I've tried playing with the altar of skulls as zombie-necromancer (fighting against the Druid). I think it probably works better in a skeleton build than a zombie build, since you then have more cards that can support it. The major strength of the altar is its skeleton subclass, which lets you use the necromancer's excellent skeletal reconstruction spells to repair it. Since it has lifebond, this basically allows the necromancer to regenerate under deathlock.
Based on my experience as necromancer, I think you want to limit yourself to one of the three major necromancer conjurations (the altar, the ziggurat, or the graveyard). I prefer zombie builds, and I find that the ziggurat works best with that strategy, so I have dropped the altar in favor of the ziggurat in my own deck.